A fuel cell is a mechanism in which energy generated in production of water by reaction of hydrogen and oxygen is electrically extracted. Fuel cells are expected to come into wide use as clean energy sources because they have high energy efficiency, and discharge only water. Among them, polymer electrolyte fuel cells are expected to be used as power sources for fuel cell vehicles.
An electrode to be used for a polymer electrolyte fuel cell is disposed so as to be sandwiched between two separators in the polymer electrolyte fuel cell. Such an electrode has a structure in which on both sides of a polymer electrolyte membrane, a catalyst layer is formed on the surface of the polymer electrolyte membrane, and a gas diffusion layer is formed outside the catalyst layer. As an individual member for forming a gas diffusion layer in an electrode, a gas diffusion electrode is circulated. The gas diffusion electrode is required to have performance such as, for example, gas diffusibility, electrical conductivity for collecting electricity generated in the catalyst layer, and water removal performance for efficiently removing water generated on the surface of the catalyst layer. For obtaining such a gas diffusion electrode, an electrically conductive porous substrate having both gas diffusibility and electrical conductivity is generally used.
Specific examples of the electrically conductive porous substrate that is used include carbon felts carbon papers and carbon cloths. Among them, carbon papers are most preferable from the viewpoint of mechanical strength etc.
In addition, the fuel cell is a system in which energy generated at the time when hydrogen and oxygen react with each other to produce water, and therefore when an electrical load increases, i.e. a current to be extracted outside the cell increases, a large amount of water (water vapor) is generated. When at a low temperature, the water vapor is condensed into water droplets, so that pores of the gas diffusion electrode are closed, and the amount of a gas (oxygen or hydrogen) supplied to the catalyst layer decreases. When all the pores are ultimately closed, power generation is stopped (this phenomenon is referred to as flooding).
The gas diffusion electrode is required to have water removal performance so that occurrence of the flooding is inhibited as much as possible. As means for improving the water removal performance, hydrophobicity is normally improved using a gas diffusion electrode substrate with an electrically conductive porous substrate subjected to a hydrophobic treatment.
When an electrically conductive porous substrate subjected to a hydrophobic treatment as described above is used directly as a gas diffusion electrode, condensation of water vapor leads to generation of large water droplets because the substrate has a coarse fiber, and thus flooding easily occurs. Thus, a coating liquid in which electrically conductive fine particles of carbon black etc. are dispersed may be applied, dried and sintered to provide a layer called as a microporous layer on an electrically conductive porous substrate subjected to a hydrophobic treatment. It is known that a fluororesin is added as a hydrophobic agent in the microporous layer for imparting hydrophobicity to the microporous layer (e.g. Patent Documents 1, 2 and 3). As a role of the microporous layer, mention is made of, in addition to that described above, an effect of preventing penetration of the catalyst layer into a coarse gas diffusion electrode substrate (e.g. Patent Document 4), and reducing coarseness of the electrically conductive porous substrate.
As the hydrophobic agent, a fluororesin is suitably used because hydrophobicity is preferably as high as possible. In particular, PTFE (polytetrafluoroethylene), FEP (ethylene tetrafluoride-propylene hexafluoride copolymer) or the like, which gives particularly high hydrophobicity, is preferably used. Such a fluororesin is commercially available normally in the form of a dispersion obtained by dispersing the fluororesin in an aqueous dispersion medium with a surfactant. Aqueous coating is preferable from the viewpoint of reduction of an environmental load.
On the other hand, power generation performance under operation conditions at a high temperature is also required. The electrolyte membrane is easily dried at a high temperature. Thus, the ion conductivity of the electrolyte membrane is reduced, leading to deterioration of power generation performance (this phenomenon is referred to as dry-out).
It is effective to control the distribution of diameters of pores in the gas diffusion electrode for preventing the flooding and dry-out. These techniques are described in, for example, Patent Document 5.